Filled a rental in Paris with gas oil once when not thinking. The gas engines are protected in the US with smaller nozzles but never dawned on me that it worked the other way with no protection. Years ago truckers would add a little gas to the diesel tank in the winter to prevent gelling. I suspect your friend had some fuel already so the gas mixed. The more mix I suspect he will be ok and the extra precautions of filters and oil change should make it better. He'll never do it again. Even with the larger nozzles for diesel I always check after the one incident. The Parisian tow truck driver was telling my wife who speaks French that her husband was not very smart. The French were right about that.